1. Field of the Invention
The present invention relates to a method for operating a vehicle, the vehicle having a first axle, a second axle, means for determining slipping of at least one wheel, which is provided on the second axle, on a ground surface of the vehicle, and a braking device associated with the wheel, and a drive torque being applied to the first axle and the second axle via a central differential, and, in the event of slipping of the wheel, a braking torque required for preventing the slipping being determined.
2. Description of Related Art
Methods of the above type are known from the related art. They are used for operating vehicles which have more than one driven axle. The vehicle may thus, for example, also be an all-wheel-drive vehicle, in which all axles or wheels of the vehicle are driven. The vehicle for which the described method may be used has the first axle and the second axle, which are both drivable. For this purpose, the axles may be coupled to one another via the central differential and may also have the drive torque applied thereto. Means, using which it may be determined whether the wheel is slipping on the ground surface of the vehicle, are associated with the wheel or the wheels of the second axle. If the wheel is slipping, the wheel may not transmit any force to the ground surface and therefore may not contribute to accelerating or braking the vehicle. For this reason, driver assistance systems, for example, in the form of a so-called electronic stability program (ESP), are used, which brake the wheel as soon as the slipping is determined. The braking device is associated with the wheel for the braking thereof. A braking torque or a braking force, which is to act on the wheel, may be set on the braking device. For this purpose, the braking torque which is required for preventing the slipping when the slipping occurs is initially determined.
In methods known from the related art, this required braking torque is set on the braking device. In this way, the slipping of the wheel is prevented, so that propulsion of the vehicle may still be ensured, even on a slippery ground surface. Such a method must be used in particular if the axle has a differential. The torque supplied by the central differential to the axle is distributed to the wheels of the axle via the differential. In a normal differential transmission, i.e., not a self-locking differential, half of the torque output at the differential is transmitted to each of the wheels. The transmittable drive torque is limited by the wheel having the worse traction. While one of the wheels slips on the ground surface, the other of the wheels may also not transmit torque. For this reason, means for determining the slipping and the braking device are provided. If the slipping wheel is braked, i.e., a braking torque is applied thereto, at least a force which corresponds to this braking torque may be applied to the ground surface via the other wheel. A part of the drive torque is lost, anyhow, since the braking torque must be applied to the wheel. Therefore, if at least one wheel slips on the ground surface of the vehicle, a part of the drive torque is not available for the propulsion of the vehicle.